Field of the Disclosure
This disclosure relates to a secondary or rechargeable battery, specifically to modifying a battery's electrode matrix for use in non-lithium metal ion applications.
Background
Developing high energy, high power, and safe batteries is of great significance so as to address the society's energy needs, such as distributed power sources, electric vehicles, and devices that handle large amounts of power. Among current battery techniques, lithium-ion batteries featuring the highest energy and power density have dominated the portable electronics market. When lithium-ion batteries are to be scaled up, however, the availability of lithium will also become a limitation. The global production of lithium can presently only satisfy half the need to convert the annually produced >60 million cars into plug-in hybrid electric vehicles (EVs) which are powered by both combustion engines and rechargeable lithium batteries. With the continued adoption of EV's, the lithium supply deficit is expected to worsen because these vehicles carry several times more batteries onboard. Although lithium reserves in sea water are rich, exploitation from the sea is presently too expensive to constitute a significant portion of lithium production.
Recent efforts to develop scalable high-energy batteries have turned attention to non-lithium techniques such as room-temperature rechargeable magnesium and sodium batteries which work in a similar way as lithium-ion batteries. Magnesium and sodium are earth abundant elements and are cheaply produced in huge amounts as shown in Table 1.
TABLE 1Comparison of Key Parameters ofLithium, Magnesium, and SodiumLithiumMagnesiumSodiumGravimetric Capacity (mAh g−1)386122051166Volumetric Capacity (mAh cm−3)206638331128Potential (V vs NHE)−3.04−2.372−2.71Global Production (kg yr−1)2.5 × 1076.3 × 1091010(very low)(high)(high)Price (carbonate; $ ton−1)5000600200Mn+ Radius (Å)0.680.650.95Polarization Strength (105/pm−2)21.647.311.1
Generally, the electrodes based on light-weight multivalent metals such as magnesium and aluminum provide some advantages over the conventional lithium. For example, they may offer up to seven times higher volumetric specific capacity than lithium-ion battery anodes, including graphite and Li4Ti5O12. In addition, their redox potentials are 0.7-1.4 V higher than lithium, implying potentially better safety; but not too high (e.g. the redox potential of aluminum is lower than the popular anode Li4Ti5O12) so that the theoretically achievable working potential is not compromised. Studies on the electrochemical deposition of magnesium showed that magnesium can be plated in a uniform dendrite-free manner and will serve as a safe anode material. Rechargeable magnesium batteries are therefore regarded as a potentially low-cost, ultra-high energy, and safe technology for energy storage.
Rechargeable batteries based on non-lithium metals share similar chemistry and fabrication techniques as those for rechargeable lithium batteries, while possessing the advantages of lower costs and better safety. However, most materials used for non-lithium metal storage have met with limited intercalation extent and inferior reversibility. A wide range of intercalation compounds have been screened for magnesium storage, including layered transition metal chalcogenides, transition metal oxides, and polyanionic magnesium salts. All these categories of compounds are established intercalation hosts for lithium battery cathodes. However, when they are directly used in the bulk form as cathodes, only Cheveral phase chalcogenides have exhibited practical magnesium intercalation. Currently, other compounds show continuous capacity fade after the initial activation stage. For oxides, no practical cycling stability has been reported and, to date, there is no cathode material exhibiting practical energy density and cyclability suitable for electrochemical storage of multivalent metal ions. For aluminum batteries, only V2O5 and TiO2 have been attempted as cathodes but, neither has exhibited practical energy density. Studies on sodium-ion batteries have also revealed intercalation chemistry that is different from their lithium-based counterparts. Many more plateaus are observed in the charge-discharge curves for electrochemical sodium intercalation, implying complex multi-phase reactions with frequent structural transformation which are detrimental to cycling stability.
Generally, the aforementioned disadvantages associated with the intercalation of non-lithium metal cations appear to be related to the fact that all these cations are larger or more polarizing than the Li+ ion. Compared to the Li+ ion, the Na+ ion has the same charge number but a significantly larger ionic radius (0.95 Å, cf 0.68 Å for Li+). As a result of steric effects, the Na+ ion exhibits sluggish intercalation/diffusion kinetics in frameworks commonly employed for Li storage. The Mg2+ ion has a similar ionic radius (0.65 Å) to Li but double the charge number, hence exhibiting high polarizing ability. The strong interaction of the multivalent Mg2+ ion with the negatively charged atoms in the host material makes the diffusion of Mg2+ difficult.